This invention relates to a liquid crystal display device having non-linear type material sandwiched between the liquid crystal and the liquid crystal driving electrodes.
Conventionally, a liquid crystal display device which is a compact, light and electrically low-consuming display device has been in practical use. Recently, in order to increase the displaying information amount of this liquid crystal display device, the following three types of liquid crystal display devices are known; a MOS type liquid crystal display device utilizing a silicon single crystal substrate; a Thin Film Transistor liquid crystal display device with a semi-conductive layer formed on glass substrates; a MIM type liquid crystal display device utilizing a non-linear element composed of Metal-Insulator-Metal. As the MOS type liquid crystal display device utilizes silicon single crystal at its substrate, it is not possible to enlarge its size. The Thin Film Transistor liquid crystal display device has some possibilities of enlarging its size, but it has such defects as, being necessary to form more than five layers of thin film layers, patterning is necessary, the picture element deficiency rate is high, and the cost is expensive. In contrast to the above two types, the MIM type liquid crystal display device has a relatively simple structure, having a possibility of enlarging its size. FIG. 1 is a diagram of a circuit wherein the display panel is driven in an X-Y matrix mode and, the display panel has the conventionally known non-linear element composed of Metal-Insulator-Metal which is serially connected with the liquid crystal. Numeral 21 is a line or column electrode group, numeral 22 is a row electrode group, usually composed of 200 to 1,000 columns and rows. At each of the intersections of X-Y electrodes, liquid crystal 23 and a non-linear-resistive element 24 are formed. This type of display device is driven with a method called multiplex driving method. In this driving method, assuming the electrical voltage to be applied to the displaying or selected picture element as V.sub.S, the electrical voltage to be applied to the non-displaying or non-selected picture element as V.sub.NS, the driving margin can be expressed as the following formula: ##EQU1##
n=division number (X the number of electrodes)
a=bias number (generally 1/3 to 1/4)
It can be understood that the display lighting voltage V.sub.S and the display eliminating voltage V.sub.NS of the liquid crystal gradually approach to each other, and, the driving marging becomes close to 1 as the division number n increases to obtain as much display picture elements as possible. Thus the liquid crystal needs to stand up or respond as fast as possible. But with the present liquid crystal, the division number n is only about 100, and so the liquid crystal cannot stand up or respond immediately. Therefore, to improve the standing up or respond characteristic of this liquid crystal, non-linear-resistive elements are serially connected to the liquid crystal.
FIG. 2 shows the characteristic of applied voltage corresponding to the transmission factor of the conventional liquid crystal. Graph 25 is the usual characteristic of twist nematic type liquid crystal, and graph 26 shows its characteristic when a Metal-Insulator-Metal non linear element is serially connected to the twist nematic type liquid crystal. In this case, the standing up or response of the liquid crystal becomes very fast, the threshold voltage V.sub.TH shifts to the high voltage side, and thus very large driving margin can be obtained.
FIG. 3 is a sectional diagram showing a conventionally known non-linear-resistivity element formed on liquid crystal panels. Numerals 27, 28 in FIG. 3 are upper and lower transparent substrates, 29 is liquid crystal, 30 is metal tantalum, 31 is a insulative layer of tantalic pentoxide (Ta.sub.2 O.sub.5) formed by the anodic oxidation of metal tantalum, and 32 is a transparent electrode for picture element display. This type of non-linear-resistive elements is comprised of thin insulative films, and the electric current passing through these elements are called either Poole-Frenkel current, or Fowler-Nordheim tunnel current. To pass these currents, the thickness of the insulative layer must be made extremely thin, of about 50 to 400 .ANG.. Non-linear-resistive element and liquid crystal are serially connected, and, to drive the selecting point, the electric charge is poured into the liquid crystal layer through the non-linear-resistive element. In the case of erasing the display, the electric charge disappears through the resistance of the liquid crystal. The driving is conducted by multiplex driving.
To conduct smoothly the displaying and erasing operations of the liquid crystal device which utilizes this type of non-linear-resistive element, the non-linear-resistive element must be provided with the following characteristics; assuming the capacity of non-linear-resistive element of one picture element as C.sub.MIM, and the capacity of the liquid crystal as C.sub.LC, at least C.sub.MIM &lt;C.sub.LC ; assuming the ON resistance of the non-linear-resistive element of one picture element as R.sub.ON, and the resistance of the liquid crystal as R.sub.LC, approximately, R.sub.ON =R.sub.LC /30.
To satisfy the above conditions, the dimension of the non-linear-resistive element must be less than 20 .mu.m.sup.2, and the highest electric current to pass through the non-linear-resistive element must be about 1 A/cm.sup.2. The driving the liquid crystal is matrix driving, and the electric field that is applied to the picture element is alternating voltage. Generally, when 1 A/cm.sup.2 of electric current is repeatedly passed through the insulative layer, the breakdown of the insulator may occur when 10.sup.4 to 10.sup.7 times current passes through. Therefore, there is a problem in the life-time of the insulator. Also, when utilizing Ta.sub.2 O.sub.5 as non-linear-resistance element, because the layer is so thin less than 400 .ANG., and also the specific inductive capacity is so high as more than 10, the size of the non-linear-resistive element must be established to be less than 20 .mu.m.sup.2. Therefore, when forming a large size display panel of more than 20 cm.sup.2, patterning with very high accuracy must be conducted, and it causes the decline of manufacturing yield, and high cost.